Fata Morgana seen on Norwegian coast: Just the hardly visible crest is on real position.
Image credit: Gerd A.T. Mueller [CC BY-SA 3.0], via Wikimedia Commons

When people accuse you of building castles in the air, they are not usually congratulating you on an incredible engineering feat, but more likely trying to bring you back down to earth with a thud. Synonymous with daydreams, pipe dreams, and all other dreams unlikely to come to fruition, castles in the air are at best a hopeful vision, and at worst, a hopeless illusion.

Although the phrase “castles in the air” (the original phrase was “castles in Spain”) is most often used to describe imaginary constructions, it can also be used to describe a very real optical phenomenon—the fata morgana effect—in which different levels of hot and cold air distort the appearance of objects on the horizon to make them look like, well, castles in the air.

Tempting Fata

Fata Morgana is the Italian name for Morgan le Fay, the half-sister of King Arthur in Arthurian legend. Reputedly a sorceress and able to change shape at will, Morgan le Fay was sometimes said to live below the sea in a crystal palace that could also rise above the surface. The fata morgana effect was so named for the superstitious belief among sailors that she created illusory visions to lure men into a false port and to their death. The term first entered English usage in 1818, when it was used to describe an occurrence of the phenomenon in the Strait of Messina, a narrow body of water between Sicily and the region of Calabria in southern Italy.

Technically, fata morganas are a type of mirage, related to those visions of water in the desert, or less exotically, to those seeming pools of water on the highway on a hot day. However, the latter two are examples of inferior mirages, while fata morganas are classified as superior mirages. It’s not that fata morganas are inherently better than the others; the difference lies in the way each mirage is produced.

Refract Up

Although the word mirage is derived from the French verb se mirer, meaning “to be reflected,” a more apt description of a mirage is that it is refracted. As light passes through layers of air with varying densities (density being determined by factors such as pressure and temperature), it bends, or more specifically, refracts, according to each layer’s characteristics.

In the case of inferior mirages, light bends upwards when it moves from a denser layer of cold air into a less dense layer of hot air, like that created above a highway on a hot day. As light hits the surface of the road and bends upwards, it looks to our eyes as if we are seeing a reflection in the road of what is just above it—in this case, the blue sky. This is because we perceive that light travels in a straight line to our eyes, even when that is factually not so.

Lake Superior

A superior mirage is the reverse of this; what we perceive to be higher in the sky is actually lower to the ground. Light is bent downwards when it hits a layer of cold air, making it appear as if what is below our sight line is actually straight ahead or above us because we are seeing the inverted image of what is on the horizon projected above it. This can be further complicated when there are multiple layers of hot and cold air, creating a highly distorted image as the light refracts through them.

Superior mirages occur wherever the surface temperature is colder than the air above it, usually over bodies of water and areas with ice or snow on the ground. The term fata morgana is most often used to describe superior mirages occurring over water. In these instances, objects on the horizon, such as ships, islands, cliffs, or icebergs, appear taller than they are because their inverted image is reflected above or superimposed on them. This elongation of objects on the horizon may make it appear as if there are turrets or towers rising up from the water, leading to the description of fata morganas as castles in the air.

As this effect can occur with ships, making them look higher above the horizon than they are, some have speculated that this is the origin of the Flying Dutchman legend, in which a ghostly ship is doomed to sail the seas for eternity.

There are many other types of superior mirages; one of them, the fata bromosa, or “fairy fog,” is created under the same conditions as the fata morgana, but has a different appearance. It appears as a bank of fog, with varying degrees of brightness, but without the fine detail of the fata morgana.

Fata Complete

Since its introduction into regular usage, the term fata morgana has come to mean more than just an optical phenomenon; although it has kept its original meaning of referring to something that is illusory, its use has been expanded throughout popular culture. It provided the title for a Henry Wadsworth Longfellow poem, an 1868 polka by Johann Strauss, and an Agatha Christie crime novel. It’s the name of a French publishing house, a character in Sergei Prokofiev’s opera, The Love for Three Oranges, and a film by Werner Herzog composed solely of desert landscape images.

The enduring popularity of the term shows how compelling it is as an idea—that there are mysterious phenomena, benign or malevolent, that are beyond our understanding. Or it may be that we continue to be enamored of our castles in the air, despite the knowledge of their illusory nature, as the last stanzas of Longfellow’s poem conclude:

So I wander and wander along,
And forever before me gleams
The shining city of song,
In the beautiful land of dreams.

But when I would enter the gate
Of that golden atmosphere,
It is gone, and I wonder and wait
For the vision to reappear.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on July 24, 2006.